Increase Lines Width in Viewport
The 'width outline' option found in themes adjusts the width of the outline drawn around selected objects. There is no option to adjust the width of lines when drawing objects in the 3dview. There is a pixel mode setting that will allow lines in the 3dview (and all other interface items) to be drawn at double width, this is intended for the Apple's retina displays. You may also want to adjust the DPI setting to control the interface outside the 3dview.

The other option involves altering blenders source code. I don't think there is one location to adjust the line size, you may have to adjust the drawing of each object type, you could setup a preference that is then used in each place. The code you would need to change should be contained within editors/space_view3d/.